1. Field of the Invention
The present invention relates to rewritable non-volatile memory, and more particularly to non-volatile semiconductor memory that prevents unauthorized reading.
2. Description of the Related Art
Rewritable non-volatile semiconductor memory is widely used as memory for storing programs and data (hereinafter “contents”). Since the contents are retained even when the power is off, such memory is widely used in portable telephones, portable information terminals and the like.
Users who purchase non-volatile memory write contents to be stored to the memory, and then use a program-protect or erase-protect function to place the memory in a protected state so that the contents are not mistakenly erased or altered thereafter. Such a protect function can prevent the modification of stored contents.
The contents stored to memory, such as a game program, for example, have their own intrinsic value. Consequently, the unauthorized copying of such contents is undesirable. Conventional non-volatile memory, however, has the abovementioned write-protect function, but does not have a function to prohibit reading. This is because the functioning of the memory requires the stored contents to be freely read. As a result, even contents having value may be read without authorization and copied to other memory.
For example, in a game device having memory in which a game program is stored, the game program may be more valuable than the device. In such a case, if the game program is copied to other memory through unauthorized copying, it may become possible to manufacture the game device at a low cost, and damage to an authorized manufacturer may be great. Consequently, a function to prevent unauthorized reading of contents in a memory is required.